These are mostly all demonology spec, but here are some random thoughts of warlock talents/skills I'd love to see...
• Improved Ritual of Doom - Still takes 4 other party members to summon a doomguard. Only difference is make it like the other summons. As in, it lasts until killed, unsummoned or player dies, like any other summoned demon. No need to enslave.
To make it "fair", since the doomguard is rather powerful and I'm sure some users may complain, increase the cool down time to 1 day. That means 1 doomguard a day. If you lose it, it's gone until the next day.
• "Capture" a demon. Would be sort of like Enslave Demon, but makes the target demon into a demon to summon. Similar to a hunter's ability to get new pets, this would allow the warlock to get 1 demon summon of their choosing, rather than using one of the normal demons we learn to summon. Obviously this won't work on all demons, like the very powerful ones. It'll be more like a "fun" talent to get imps or voidwalkers that just look different than the normal ones warlocks summon. Only can use this on 1 demon. If you want to change the demon you picked, you need to go to a warlock trainer to get rid of it.
• Demonic Circle - Allow this usable between worlds/continents. Right now, if you use Demonic Circle: Summon in, for example, Daralan and then use one of the city portals, you can't teleport back by using the Demonic Circle: Teleport.
• Some sort of flying demon mount. We've got land mounts, so why not a flying one? Or maybe flying versions of the Felsteed/Dreadsteed (they'd be like the Headless Horseman's horse mount).
• Lesser Invisibility - Not like a rogues stealth. This would only work for maybe 15 seconds and would still be detectable if you get close to a mob. Anything other than walking/running would break the invisibility. Maybe a 1-2 minute cool down timer and not able to cast while in combat.
• Improved Metamorphosis - Change lasts until player shifts back to normal on their own. Maybe make it cost 1 soul shard every 5 minutes, to remain in the form.